The HM-101 comes with two fixed level outputs. The Line-level output for feeding a separate headphone amplifier, and a Headphone output for direct use with headphones. There is no volume control knob on the HM-101 and so volume is controlled from the computer. While there may be debates to this method, I find that in real-world usage, at least with my Macbook Air, controlling volume through the computer is far more convenient than with a volume knob. The Mac keyboard comes with dedicated keys on the keyboard for mute, volume up and down functions, and so your hand never have to leave the keyboard. If you find the volume of the next song to be too loud, you just hit the volume down key on the keyboard till you get the perfect loudness level.
A while ago Benchmark Audio wrote that the volume control on newer iTunes version (version 7 and up) may be used without any worries in introducing distortions into the sound. In this case I’m not using the iTunes’ volume control, but rather the OSX volume control, simply due to the convenience of having the dedicated keys, but even then I can’t really pick up any audible distortions or artifacts in the sound.
Of course the lack of a manual volume control knob may seem odd to some people, but I can assure you that there is nothing but benefits by having the volume controlled in the digital domain. Any distortions done in the ones and zeros are probably going to be less noticeable than the distortions caused by a mechanical potentiometer, and in fact I think this is one of the reason to why Hifiman was able to extract so much performance out of a simple PCM2702 chip. Another benefit of the digital volume control is that there is zero channel imbalance at any volume level. And by using both the iTunes and the OSX volume control, I can virtually have the volume control range set to any level I want.
Conclusion on the comparisons
In comparison to the recent products that I’ve reviewed, the Fiio E10 and the Ibasso D-Zero, given the prevalence of strong beats and bass in today’s music, I think most people would be happiest with the D-Zero as it’s just best in that area. The Fiio E10 was pretty good with bass and also mids. The HM-101 is the most limited of the three, where I find it to excel mostly in the mids. However it happens to be my favorite in terms of an analog-like sound reproduction and the most lifelike soundstage ambiance. So, if you happen to be a big fan of midrange and soundstage, you really should give the HM-101 a try.
I think the beauty in the HM-101 is in the simplicity of the set up, the volume control-less operation (done on the computer), and the superb soundstage performance. As I mentioned above on the paragraph talking about bass, moving to a separate amp (this time the E6 and the D-Zero) would give me better punch in the bass. But you also lose things like the transparency and the ambiance in the soundstage, and the sweet midrange body which tends to be a little flat on a separate external amp. So I do think that the HM-101 really works best as it is. After all it’s not intended to be an ultimate USB DAC/Amp solution, but more like a good quality USB DAC at an affordable price. You know, a USB DAC to keep in the office and enjoy music from without having to break the bank and build a very expensive set up around. And I think it succeeded in becoming such a product. Not only does it pass the “good quality” mark, but I happen to really enjoy its musical presentation.